Such devices are known from the field of gas chromatography or chemiluminescence analysis and are used, for example, to determine the content of nitrogen monoxide in exhaust gases.
Oxygen and nitrogen dioxide are formed during the spontaneous reaction of nitrogen monoxide and ozone, with a part of the nitrogen dioxide produced being in an excited electron state. The molecules spontaneously emit this excessive energy in the form of optically measureable fluorescent radiation that is proportional to the concentration of nitrogen monoxide in the sample gas.
Since the reaction described applies only to the nitrogen monoxide molecules, the nitrogen dioxide parts in the exhaust gas are reduced to nitrogen monoxide before entering the analysis chamber. This is effected in a thermal or thermal/catalytic converter at temperatures above 200° C. Ozone for the reaction is further produced from oxygen in an ozonisator. The volume flow of the exhaust gas is maintained constant in order to achieve a light yield that is proportional to the nitrogen oxide concentration. The exhaust gas is correspondingly conducted into the analysis chamber via capillaries, the pressure and the temperature of the exhaust gas being controlled. The radiation is converted into an electric signal by means of a cooled photomultiplier that serves as a detector. This signal is supplied to the evaluation unit as a measure of the nitrogen oxide emission.
It has been found that particularly good results are achieved, for example, when high temperatures of above 100° C. are generated in the analysis chamber and cool temperatures prevail in the area of the detector. Moreover, due to diffusion or absorption, losses often occur during the measurement of the optical radiation.
DE-OS 27 16 284 describes a device for determining gaseous components in which a heated reaction chamber is provided which has an inlet for ozone and an inlet for the sample gas. The chamber is thermally separated by an asbestos ring from a photoelectric measuring cell arranged in a cooled housing. Two cylinders coated with a reflective material are arranged between the reaction chamber and the measuring cell, and two quartz glass windows mounted in a ring are arranged between the cylinders.
This device has the disadvantage that no full thermal separation is achieved with the two quartz glass windows. Radiation losses further occur due to absorption at the quartz glass windows since these are arranged in a ring. It is also doubtable whether the coating of the cylinders which is constantly exposed to the gas flow will, over the long term, fulfill its task of possibly providing a total reflection. Deposits of ammonia on the walls of the analysis chamber cannot be prevented.